Sensors for detecting explosives typically employ cantilever resonators driven by electrostatic forces or electromagnetic forces and capacitive pickoff. These sensors typically operate by flowing the sample volumes of air over the sensors directly over the resonant cantilever. These motional displacements of the cantilever sensor designs have several shortcomings. First, the cantilever sensors displace prodigious amounts of gas relative to their size because the cantilever is thin and flat relative to the direction of motion. At atmospheric pressure, this would result in a very low Q, the measure of energy consumed per cycle versus the energy stored per cycle. Where narrow gaps are used between the resonating cantilever and the electrostatic drive, sense electrode, squeeze film damping becomes very significant. Even at pressures wherein the mean free path of the gases is on the order of the gap, the effect on Q is significant. The Q of the resonator is a primary indicator of the detection threshold of the resonator. The higher the Q, the better the sensitivity of the resonator.
Additionally, the direct impingement of the sample air flow on the sensor surface has the degrading factor of potentially collecting particulate contamination of the mass-spring-damper system. Particulate contamination commonly imparts a frequency shift error well in excess of the vapor detection level.
The resonant cantilever described in U.S. Pat. No. 7,521,257 detects chemical species by the change in amplitude of resonance. This approach suffers from inaccuracies due to changes in Q or pressure. The teeter-totter resonator described in U.S. Pat. No. 6,820,469 operates with motion out of plane that displaces a sizeable volume of gas each cycle resulting with a relatively low Q. The electromagnetically driven resonator described in U.S. Pat. No. 6,668,627 utilizes a thin flat plate moving out of plane resulting in displacement of a large amount of gas per cycle resulting in a low Q. The resonant plate approach described in U.S. Pat. No. 7,305,883 also utilizes out of plane motion resulting in a sub-optimal Q. Patents 2005/0101026 A, WO/2008/005096, and 2009/0246881 A1 teach about optical detection methods without the properties of mass change for gravimetric sensing.